Three dimensional NAND device with birds beak containing floating gates and method of making thereof

ABSTRACT

A method of making a monolithic three dimensional NAND string including forming a stack of alternating layers of a first material and a second material over a substrate. The first material comprises an electrically insulating material and the second material comprises a semiconductor or conductor material. The method also includes etching the stack to form a front side opening in the stack, forming a blocking dielectric layer over the stack of alternating layers of a first material and a second material exposed in the front side opening, forming a semiconductor or metal charge storage layer over the blocking dielectric, forming a tunnel dielectric layer over the charge storage layer, forming a semiconductor channel layer over the tunnel dielectric layer, etching the stack to form a back side opening in the stack, removing at least a portion of the first material layers and portions of the blocking dielectric layer.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/843,835, filed Jul. 8, 2013 and U.S. Provisional Application No.61/845,038, filed Jul. 11, 2013, the contents of which are herebyincorporated by reference in their entirety.

FIELD

The present invention relates generally to the field of semiconductordevices and specifically to three dimensional vertical NAND strings andother three dimensional devices and methods of making thereof.

BACKGROUND

Three dimensional vertical NAND strings are disclosed in an article byT. Endoh, et. al., titled “Novel Ultra High Density Memory With AStacked-Surrounding Gate Transistor (S-SGT) Structured Cell”, IEDM Proc.(2001) 33-36. However, this NAND string provides only one bit per cell.Furthermore, the active regions of the NAND string is formed by arelatively difficult and time consuming process involving repeatedformation of sidewall spacers and etching of a portion of the substrate,which results in a roughly conical active region shape.

SUMMARY

An embodiment is drawn to a method of making a monolithic threedimensional NAND string including forming a stack of alternating layersof a first material and a second material over a substrate. The firstmaterial comprises an electrically insulating material and the secondmaterial comprises a semiconductor or conductor material. The methodalso includes etching the stack to form a front side opening in thestack, forming a blocking dielectric layer over the stack of alternatinglayers of a first material and a second material exposed in the frontside opening, forming a semiconductor or metal charge storage layer overthe blocking dielectric, forming a tunnel dielectric layer over thecharge storage layer, forming a semiconductor channel layer over thetunnel dielectric layer, etching the stack to form a back side openingin the stack, removing at least a portion of the first material layersand portions of the blocking dielectric layer through the back sideopening to form back side recesses between the second material layersand oxidizing regions of the charge storage layer adjacent the back siderecesses to form discrete charge storage regions.

Another embodiment is drawn to a method of making a monolithic threedimensional NAND string including forming a stack of alternating firstand second layers over a substrate. The first layers comprise anelectrically insulating composite layer comprising a silicon nitridelayer between silicon oxide layers and the second layers comprise asemiconductor or conductor material. The method also includes etchingthe stack to form a front side opening in the stack, forming a blockingdielectric layer over the stack of alternating first and second layersexposed in the front side opening, forming a charge storage layer overthe layer of high work function material, forming a tunnel dielectriclayer over the charge storage layer, forming a semiconductor channellayer over the tunnel dielectric layer, etching the stack to form a backside opening in the stack, removing at least a portion of the siliconnitride layer between silicon oxide layers to form back side recessesbetween adjacent second layers, removing portions of the blockingdielectric layer exposed in the back side recesses and forming discretecharge storage regions.

Another embodiment is drawn to a monolithic three dimensional NANDstring including a semiconductor channel, at least one end portion ofthe semiconductor channel extending substantially perpendicular to amajor surface of a substrate, a plurality of control gate electrodesextending substantially parallel to the major surface of the substrate.The plurality of control gate electrodes comprise at least a firstcontrol gate electrode located in a first device level and a secondcontrol gate electrode located in a second device level located over themajor surface of the substrate and below the first device level. Also ablocking dielectric located in contact with the plurality of controlgate electrodes, a plurality of vertically spaced apart charge storageregions located in contact with the blocking dielectric. The pluralityof vertically spaced apart charge storage regions comprise at least afirst spaced apart charge storage region located in the first devicelevel and a second spaced apart charge storage region located in thesecond device level and a portion of the first and second charge storageregions comprises a bird's beak shape. And a tunnel dielectric locatedbetween each one of the plurality of the vertically spaced apart chargestorage regions and the semiconductor channel.

Another embodiment is drawn to a method of making a monolithic threedimensional NAND string including forming a stack of alternating layersof a first material and a second material over a substrate. The firstmaterial comprises an electrically insulating material and the secondmaterial comprises a semiconductor or conductor material. Also, etchingthe stack to form a front side opening in the stack, forming a blockingdielectric layer over the stack of alternating layers of a firstmaterial and a second material exposed in the front side opening,forming a charge storage layer over the blocking dielectric, forming atunnel dielectric layer over the charge storage layer, forming asemiconductor channel layer over the tunnel dielectric layer, etchingthe stack to form a back side opening in the stack, removing at least aportion of the first material layers through the back side opening toform back side recesses between the second material layers, forming aprotective layer on portions of the second material layers exposed inthe back side recesses, after forming the protective layer, removingportions of the blocking dielectric layer exposed in the back side therecesses through the back side opening and forming discrete chargestorage regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are respectively side cross sectional and top crosssectional views of a NAND string of one embodiment. FIG. 1A is a sidecross sectional view of the device along line Y-Y′ in FIG. 1B, whileFIG. 1B is a side cross sectional view of the device along line X-X′ inFIG. 1A.

FIGS. 2A-2B are respectively side cross sectional and top crosssectional views of a NAND string of another embodiment. FIG. 2A is aside cross sectional view of the device along line Y-Y′ in FIG. 2B,while FIG. 2B is a side cross sectional view of the device along lineX-X′ in FIG. 2A.

FIG. 3 is side cross sectional view of a NAND string of an embodimentwith a U-shaped channel.

FIGS. 4A-4C, 5A-5D and 6A-6D are side cross sectional views illustratingembodiments of methods of making the NAND strings illustrated in FIGS.1-3.

DETAILED DESCRIPTION

The embodiments of the invention provide a monolithic, three dimensionalarray of memory devices, such as an array of vertical NAND stringshaving selectively formed, discreet metal, semiconductor or silicidecharge storage regions. The NAND strings are vertically oriented, suchthat at least one memory cell is located over another memory cell. Thearray allows vertical scaling of NAND devices to provide a higherdensity of memory cells per unit area of silicon or other semiconductormaterial.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as asemiconductor wafer, with no intervening substrates. The term“monolithic” means that layers of each level of the array are directlydeposited on the layers of each underlying level of the array. Incontrast, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device. For example,non-monolithic stacked memories have been constructed by forming memorylevels on separate substrates and adhering the memory levels atop eachother, as in Leedy, U.S. Pat. No. 5,915,167, titled “Three DimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

In some embodiments, the monolithic three dimensional NAND string 180comprises a semiconductor channel 1 having at least one end portionextending substantially perpendicular to a major surface 100 a of asubstrate 100, as shown in FIGS. 1A and 2A. For example, thesemiconductor channel 1 may have a pillar shape and the entirepillar-shaped semiconductor channel extends substantiallyperpendicularly to the major surface of the substrate 100, as shown inFIGS. 1A and 2A. In these embodiments, the source/drain electrodes ofthe device can include a lower electrode 102 provided below thesemiconductor channel 1 and an upper electrode 202 formed over thesemiconductor channel 1, as shown in FIGS. 1A and 2A. Alternatively, thesemiconductor channel 1 may have a U-shaped pipe shape, as shown in FIG.3. The two wing portions 1 a and 1 b of the U-shaped pipe shapesemiconductor channel may extend substantially perpendicular to themajor surface 100 a of the substrate 100, and a connecting portion 1 cof the U-shaped pipe shape semiconductor channel 1 connects the two wingportions 1 a, 1 b extends substantially parallel to the major surface100 a of the substrate 100. In these embodiments, one of the source ordrain electrodes 202 ₁ contacts the first wing portion of thesemiconductor channel from above, and another one of a source or drainelectrodes 202 ₂ contacts the second wing portion of the semiconductorchannel 1 from above. An optional body contact electrode (not shown) maybe disposed in the substrate 100 to provide body contact to theconnecting portion of the semiconductor channel 1 from below. The NANDstring's select or access transistors are not shown in FIGS. 1-3 forclarity.

In some embodiments, the semiconductor channel 1 may be a filledfeature, as shown in FIGS. 2A-2B and 3. In some other embodiments, thesemiconductor channel 1 may be hollow, for example a hollow cylinderfilled with an insulating fill material 2, as shown in FIGS. 1A-1B. Inthese embodiments, an insulating fill material 2 may be formed to fillthe hollow part surrounded by the semiconductor channel 1. The U-shapedpipe shape semiconductor channel 1 shown in FIG. 3 may alternatively bea hollow cylinder filled with an insulating fill material 2, shown inFIGS. 1A-1B.

The substrate 100 can be any semiconducting substrate known in the art,such as monocrystalline silicon, IV-IV compounds such assilicon-germanium or silicon-germanium-carbon, III-V compounds, II-VIcompounds, epitaxial layers over such substrates, or any othersemiconducting or non-semiconducting material, such as silicon oxide,glass, plastic, metal or ceramic substrate. The substrate 100 mayinclude integrated circuits fabricated thereon, such as driver circuitsfor a memory device.

Any suitable semiconductor materials can be used for semiconductorchannel 1, for example silicon, germanium, silicon germanium, or othercompound semiconductor materials, such as III-V, II-VI, or conductive orsemiconductive oxides, etc. The semiconductor material may be amorphous,polycrystalline or single crystal. The semiconductor channel materialmay be formed by any suitable deposition methods. For example, in oneembodiment, the semiconductor channel material is deposited by lowpressure chemical vapor deposition (LPCVD). In some other embodiments,the semiconductor channel material may be a recrystallizedpolycrystalline semiconductor material formed by recrystallizing aninitially deposited amorphous semiconductor material.

The insulating fill material 2 may comprise any electrically insulatingmaterial, such as silicon oxide, silicon nitride, silicon oxynitride, orother high-k insulating materials.

The monolithic three dimensional NAND string further comprise aplurality of control gate electrodes 3, as shown in FIGS. 1A-1B, 2A-2B,and 3. The control gate electrodes 3 may comprise a portion having astrip shape extending substantially parallel to the major surface 100 aof the substrate 100. The plurality of control gate electrodes 3comprise at least a first control gate electrode 3 a located in a firstdevice level (e.g., device level A) and a second control gate electrode3 b located in a second device level (e.g., device level B) located overthe major surface 100 a of the substrate 100 and below the device levelA. The control gate material may comprise any one or more suitableconductive or semiconductor control gate material known in the art, suchas doped polysilicon, tungsten, copper, aluminum, tantalum, titanium,cobalt, titanium nitride or alloys thereof.

A blocking dielectric 7 is located adjacent to the control gate(s) 3 andmay surround the control gate electrode 3. The blocking dielectric 7 maycomprise a layer having plurality of blocking dielectric segmentslocated in contact with a respective one of the plurality of controlgate electrodes 3, for example a first dielectric segment 7 a located indevice level A and a second dielectric segment 7 b located in devicelevel B are in contact with control gate electrodes 3 a and 3 b,respectively, as shown in FIG. 3.

The monolithic three dimensional NAND string also comprise a pluralityof discrete charge storage regions or segments 9 located between theblocking dielectric 7 and the channel 1. Similarly, the plurality ofdiscrete charge storage regions 9 comprise at least a first discretecharge storage region 9 a located in the device level A and a seconddiscrete charge storage region 9 b located in the device level B, asshown in FIG. 3.

The discrete charge storage regions 9 may comprise a plurality ofvertically spaced apart, conductive (e.g., metal such as tungsten,molybdenum, tantalum, titanium, platinum, ruthenium, and alloys thereof,or a metal silicide such as tungsten silicide, molybdenum silicide,tantalum silicide, titanium silicide, nickel silicide, cobalt silicide,or a combination thereof), or semiconductor (e.g., polysilicon) floatinggates, such as a floating gate comprising a layer of polysilicon or alayer of polysilicon with a thin layer of a high work function material6 (e.g. a material with a higher work function than the polysilicon oramorphous silicon regions 9), such as ruthenium or titanium nitride, asshown in FIG. 4A.

The tunnel dielectric 11 of the monolithic three dimensional NAND stringis located between each one of the plurality of the discrete chargestorage regions 9 and the semiconductor channel 1.

The blocking dielectric 7 and the tunnel dielectric 11 may beindependently selected from any one or more same or differentelectrically insulating materials, such as silicon oxide, siliconnitride, silicon oxynitride, or other insulating materials. The blockingdielectric 7 and/or the tunnel dielectric 11 may include multiple layersof silicon oxide, silicon nitride and/or silicon oxynitride (e.g., ONOlayers) as illustrated in more detail below.

FIGS. 4A-4C illustrate a method of making a NAND string according to afirst embodiment of the invention.

Referring to FIG. 4A, a stack 120 of alternating layers 19 (19 a, 19 betc.) and 121 (121 a, 121 b, etc.) are formed over the major surface ofthe substrate 100. Layers 19, 121 may be deposited over the substrate byany suitable deposition method, such as sputtering, CVD, PECVD, MBE,etc. The layers 19, 121 may be 6 to 100 nm thick.

In this embodiment, the first layers 19 comprise any suitablesacrificial material, such as an electrically insulating material thatmay be selectively etched relative to the second layers 121. Anysuitable insulating material may be used, such as silicon oxide, siliconnitride, silicon oxynitride, a high-k dielectric (e.g., aluminum oxide,hafnium oxide, etc. or an organic insulating material). The secondlayers 121 comprise a conducting or a doped semiconducting material thatcan function as a control gate electrode 3 of a NAND string. Forexample, layers 121 may comprise silicon, such as amorphous silicon orpolysilicon, or another semiconductor material, such as a group IVsemiconductor, including silicon-germanium and germanium. In anembodiment, layers 121 comprise p-type or n-type doped semiconductormaterials, such as heavily doped materials. The term heavily dopedincludes semiconductor materials doped n-type or p-type to aconcentration of above 10¹⁸ cm⁻³. In contrast, lightly dopedsemiconductor materials have a doping concentration below 10¹⁸ cm⁻³ andintrinsic semiconductor materials have a doping concentration below 10¹⁵cm⁻³.

The deposition of layers 19, 121, is followed by etching the stack 120to form at least one a front side opening 81 in the stack 120. An arrayof front side openings 81 (e.g., memory holes) may be formed inlocations where vertical channels of NAND strings will be subsequentlyformed. The openings 81 may be formed by photolithography and etching.The blocking dielectric (e.g., ONO or silicon oxide) 7, the chargestorage layer 9, including an optional high work function material layer6 (e.g. ruthenium or titanium nitride), the tunnel dielectric 11 and thechannel layer 1 may then be deposited in the front side opening withprocesses known in the art, such as atomic layer deposition or chemicalvapor deposition.

Next, as shown in FIG. 4B, an array of back side openings 84 are formedin the stack 120, such as by photolithography and etching. The back sideopenings may have an elongated trench shape, such as a slit trenchshape. Then, the first layers 19 are removed via the back side openings84 by selectively etching the first layers 19 compared to the secondlayers 121 to form back side recesses 62 in the stack 120 (i.e., inspaced previously occupied by the first layers 19 a, 19 b, etc). Theback side recesses 62 may be formed by selective, isotropic wet etchingor dry etching (e.g., by SiConi™ remote plasma assisted dry etchingwhich involves the simultaneous exposure of a substrate to H₂, NF₃ andNH₃ plasma by-products) which selectively etches the first layer 19compared to the second layer 121. Portions of blocking dielectric 7exposed in the back side recesses 62 are also removed during the etchingstep to expose the charge storage layer 9 or the optional high workfunction layer 6 (if provided) in the back side recesses 62. Theremaining second layers 121 form the control gate electrodes 3. The stepof removing at least a portion of the first layers 19 leaves secondmaterial control gates 3 contacting the blocking dielectric layer 7portions separated by the back side recesses 62.

Next, as illustrated in FIG. 4C, an oxidation step is performed. In thisstep, the exposed portions of the control gate electrodes 3 areoxidized. Further, the exposed portions of the charge storage layer 9 inthe back side recess 62 are oxidized. Preferably, oxidation is performeduntil the exposed portions of the charge storage layer 9 are oxidizedentirely through their thickness. That is, oxidation is performed untilthe oxidized portions 25 of the charge storage layer 9 extend to thetunnel dielectric 11. In this manner, the charge storage layer 9 issegmented to form a plurality of discrete charge storage regions orsegments (e.g. 9 a, 9 b). In an embodiment, the exposed portion of thecontrol gate electrodes 3 and the exposed portion of the charge storagelayer in the back side recesses 62 are oxidized in the same oxidationstep. If present, the exposed portions of the ruthenium layer 6 in theback side recess 62 also oxidize and are removed from the stack 120 viasublimation through the back side opening 84. For example, the oxidationmay be conducted in two steps. In the first step, the exposed portionsof the ruthenium layer 6 in the back side recess 62 are oxidized byoxygen plasma and are removed by sublimation. The exposed portions ofthe charge storage layer 9 are then oxidized in a second oxidation step.

Typically, the oxidized portions 25 of the charge storage layer 9 resultin the charge storage regions 9 a, 9 b having concave boundaries withthe oxidized portions 25 of the charge storage layer 9. That is, theboundaries of the discrete charge storage regions 9 a, 9 b may have abird's peak shape 27. In other words, the concave boundaries are locatedon the horizontal portions of the regions 9 a, 9 b having a middleportion facing inward in each of the regions 9 a, 9 b. The outerportions of regions 9 a, 9 a protrude outwardly in the verticaldirection and have a bird's beak shape having a flat surface joining acurved surface at a point or narrow tip, similar to the shape formed ina silicon substrate during the LOCOS process. Thus, the resulting firstand second charge storage regions 9 a, 9 b each preferably comprise asilicon (e.g., polysilicon) region having the bird's beak shape andregion of material 6 having a higher work function than the polysiliconregion.

After forming the discrete charge storage regions 9 a, 9 b, the backside recesses 62 may be filled with an insulating material or left asair gap insulating regions.

The remaining steps to make a NAND string may be performed as taught inU.S. Pat. No. 8,349,681 or in U.S. application Ser. No. 14/133,979 filedon Dec. 19, 2013, both of which are incorporated herein by reference intheir entirety.

FIGS. 5A-5D illustrate a method of making a NAND string according toanother embodiment. In this embodiment, the electrically insulatingfirst layers 19 comprise composite layers 19 c (e.g., first and secondcomposite layers 19 ca, 19 cb) that each include three layers, a firstlayer 31, a second layer 32 and a third layer 33. In an embodiment, thefirst layer 31 of the composite layer 19 c comprises an oxide, such asSiO₂. The second layer 32 of the composite layer 19 c comprises anitride, such as Si₃N₄. The third layer 33 of the composite layer 19 ccomprises an oxide, such as SiO₂.

The method shown in FIG. 5A includes forming the front side openings 81and deposition of the blocking dielectric (e.g., ONO) 7, the chargestorage layer 9, the optional high work function material layer 6, thetunnel dielectric 11 and the channel layer 1 into each of the front sideopenings 81, similar to the steps described above with respect to FIG.4A. Then, as illustrated in FIG. 5B, back side openings 84 are formed inthe stack 120, such as by photolithography and etching. Next, the secondlayer 32 of the composite layer 19 c is selectively removed, such as byselective etching (e.g., using hot H₃PO₄ etch), to form a back siderecess 62 between the first and third layers 31, 33 of the compositelayer 19 c. Preferably, selective etching of the second layer 32 isperformed until the blocking dielectric 7 is reached. The first andthird layers 31, 33 of the composite layer 19 c protect the exposedsurfaces of the second layers 121 that will form the control gateelectrodes 3. In this manner, a back side recess 62 with a high aspectratio (length to width) can be fabricated without excess thinning of thesecond layers 121.

As illustrated in FIG. 5C, after the second layer 32 is selectivelyremoved, the first and third layers 31, 33 are removed (e.g., byselective etching), thereby widening the back side recess 62.Additionally, as illustrated in FIG. 5C, portions of the blockingdielectric 7 (e.g., of the oxide-nitride-oxide blocking dielectric)located between the second layers 121 and exposed in the back sideopenings 62 are removed. After removing the first and third layers 31,33 and the portions of the blocking dielectric 7 located between thesecond layers, the exposed portions of the second layers 121 (which formthe control gate electrodes 3) are preferably coated with a protectivelayer 35 to protect the control gate electrodes 3 during furtherprocessing. In an embodiment, the exposed second layers 3/121 are coatedwith silicon nitride layer 35. If the second layers 3/121 compriseheavily doped silicon (e.g., polysilicon), the silicon nitride layer 35may be formed by reacting the exposed silicon with nitrogen. In thisstep, the second layers 3/121 may be thinned slightly. However, thisthinning is less than that which results from the method stepsillustrated in FIGS. 4B-4C. If desired, the protective layer 35 may beomitted, and the second layers 121 may be thicker than the first layer19 to allow for some thinning of the second layers 121 during theblocking dielectric 7 etching steps.

If the blocking dielectric 7 comprises an oxide-nitride-oxide compositedielectric, then the above described etching and coating steps may becarried out sequentially as follows. First, the outer oxide layer (i.e.,the layer facing the control gate electrodes 3) and the nitride layer ofthe blocking dielectric layer and the silicon oxide layers 31, 33 of thecomposite layer 19 are etched away in a first etching step afterremoving the silicon nitride layer 32 of the composite layer 19. Then,the protective silicon nitride layer 35 is formed on portions of thesecond material layers 3/121 exposed in the back side recesses 62. Thisis followed by a second etching step to remove the inner oxide layer(i.e., the layer facing the charge storage layer 9) of the blockingdielectric 7.

In the step illustrated in FIG. 5D, portions of the charge storage layer9, including the high work function material layer 6, if present, areexposed to oxygen. As discussed above, ruthenium forms a volatilespecies (i.e., it ruthenium sublimates) which is removed via back sideopenings 84. Further, as in the previous embodiment, the charge storagelayer 9 is oxidized entirely through its thickness to form discretecharge storage regions (e.g., intrinsic or low doped floating gates) 9a, 9 b. As discussed above, the oxidized portions 25 of the chargestorage layer 9 result in the charge storage regions 9 a, 9 b havingconcave boundaries with the oxidized portions 25 of the charge storagelayer 9. That is, the boundaries of the discrete charge storage regions9 a, 9 b may have a bird's peak shape 27. The protective layer 35protects the control gate electrodes 3 from thinning during theoxidation step. The method illustrated in FIGS. 5A-5D leaves theprotective layer 35 located on portions of the control gate electrodes 3not in contact with the blocking dielectric 7 (i.e., on the top, bottomand back sides of the control gate electrodes 3).

The discrete charge storage regions 9 a, 9 b may be formed by eitheroxidation of portions of the charge storage layer 9 exposed in the backside recesses 62, as described above, or by etching the portions of thecharge storage layer 9 exposed in the back side recesses 62, as will bedescribed below with respect to FIGS. 6A-6D.

FIGS. 6A-6D illustrate a method of forming a NAND string according toanother embodiment in which the step of forming the discrete chargestorage regions comprises removing portions of the charge storage layer9 exposed in the back side recesses 62 by etching.

As illustrated in FIG. 6A, in this embodiment, as in the previousembodiment, the composite layer 19 c includes three layers describedabove: the first (e.g., silicon oxide) layer 31, the second (e.g.,silicon nitride) layer 32 and the third (e.g., silicon oxide) layer 33.However, in this embodiment, the first layers 19 (e.g., 19 a, 19 b) maycomprise polysilicon or amorphous silicon heavily doped with at leastone of carbon or boron. The concentration of carbon or boron may be inthe range of 10¹⁹ to 10²¹ atoms/cm³.

As illustrated in FIG. 6B, the second layer 32 is selectively removedfrom the composite layer 19 c to form the back side recesses 62, asdescribed above. Then, as illustrated in FIG. 6C, the first and thirdlayers 31, 33 are removed, thereby increasing the width of the back siderecess 62. Portions of the blocking dielectric 7 exposed in the backside recesses 62 are also removed during the etching step. If layer 6 ispresent, then the portions of layer 6 exposed in the back side recesses62 may be removed by ashing using oxygen plasma.

As illustrated in FIG. 6C, the exposed portions of the second layers 121are not coated with a protective layer 35. However, as in the previousembodiment, the exposed portions of the second layers 121 may be coatedwith a protective layer 35 if desired.

FIG. 6D illustrates the next step in the method. In this embodiment,rather than forming oxidized portions 25 in the charge storage layer 9to form discrete charge storage regions 9 a, 9 b as in the previousembodiment, exposed regions of the charge storage layer 9 in the backside recesses 62 are removed by selective etching. Preferably, thecharge storage layer 9 is etched entirely through its thickness asillustrated in FIG. 6D to form discrete charge storage regions 9 a, 9 bseparated by air gaps 29.

In this embodiment, the polysilicon or amorphous silicon layers 121 aredoped with at least one of carbon or boron, while the charge storagelayer 9 is not doped with carbon or boron. Carbon doping reducespolysilicon grain size and results in fewer voids. Layer 9 may beintrinsic or lightly doped with an n-type dopant, such as arsenic orphosphorus. The different doping characteristics of layers 121 and 9allow layer 9 to be selectively etched compared to layers 121. Forexample, to the intrinsic polysilicon of layer 9 etches faster than theC and/or B doped polysilicon or amorphous silicon of layers 121 when EDP(ethylenediamine pyrocatechol) is used as the etching liquid during theselective etching of layer 9 to form discreet floating gates 9 a, 9 b.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of making a monolithic three dimensional NAND string, comprising: forming a stack of alternating layers of a first material and a second material over a substrate, wherein the first material comprises an electrically insulating material and wherein the second material comprises a semiconductor or conductor material; etching the stack to form a front side opening in the stack; forming a blocking dielectric layer over the stack of alternating layers of a first material and a second material exposed in the front side opening; forming a semiconductor or metal charge storage layer over the blocking dielectric; forming a tunnel dielectric layer over the charge storage layer; forming a semiconductor channel layer over the tunnel dielectric layer; etching the stack to form a back side opening in the stack; removing at least a portion of the first material layers and portions of the blocking dielectric layer through the back side opening to form back side recesses between the second material layers; and oxidizing regions of the charge storage layer adjacent the back side recesses to form discrete charge storage regions, wherein at least one feature selected from a group consisting of five features is employed to make the monolithic three dimensional NAND string; and the group of five features consists of: a first feature wherein oxidizing regions of the charge storage layer also oxidizes surfaces of the second material exposed in the back side recesses; a second feature wherein the blocking dielectric layer comprises a layer of silicon nitride between layers of silicon oxide; a third feature wherein the method further comprises forming a protective layer over exposed portions of the second layer in the back side recesses after forming the back side recesses; a fourth feature wherein the method further comprises forming a layer of material having a higher work function than the semiconductor or metal charge storage layer over the blocking dielectric prior to forming the charge storage layer; and a fifth feature wherein the discrete charge storage regions comprise floating gates having concave boundaries with oxidized semiconductor or metal regions of the charge storage layer, and the second material comprises polysilicon or amorphous silicon, and the polysilicon or the amorphous silicon is doped with at least one of carbon or boron and the floating gates comprise intrinsic polysilicon that etches faster than the doped polysilicon or doped amorphous silicon second material.
 2. The method of claim 1, wherein the layers of a first material comprises composite layers comprising a layer of silicon nitride between layers of silicon oxide.
 3. The method of claim 1, further comprising filling the back side recesses with an insulating material.
 4. The method of claim 1, wherein removing at least a portion of the first material layers leaves second material control gates contacting the blocking dielectric layer portions separated by the back side recesses.
 5. The method of claim 1, wherein the first feature is employed to make the monolithic three dimensional NAND string.
 6. The method of claim 1, wherein the second feature is employed to make the monolithic three dimensional NAND string.
 7. The method of claim 1, wherein the discrete charge storage regions comprise floating gates having concave boundaries with oxidized semiconductor or metal regions of the charge storage layer.
 8. The method of claim 7, wherein portions of the concave boundaries of the discrete charge storage regions comprise a bird's peak shape.
 9. The method of claim 7, wherein the second material comprises polysilicon or amorphous silicon.
 10. The method of claim 1, wherein the fifth feature is employed to make the monolithic three dimensional NAND string.
 11. The method of claim 10, wherein a concentration of the at least one of carbon or boron is in the range of 10¹⁹ to 10²¹ atoms/cm³.
 12. The method of claim 1, wherein the third feature is employed to make the monolithic three dimensional NAND string.
 13. The method of claim 12, wherein the protective layer comprises silicon nitride.
 14. The method of claim 1, wherein the fourth feature is employed to make the monolithic three dimensional NAND string.
 15. The method of claim 14, further comprising removing at least a portion of the layer of material having the higher work function through the back side opening.
 16. The method of claim 15, wherein the layer of higher work function material comprises a ruthenium or titanium nitride layer.
 17. The method of claim 16, wherein the least a portion of the ruthenium layer removed is removed by oxidation and sublimation.
 18. A method of making a monolithic three dimensional NAND string, comprising: forming a stack of alternating first and second layers over a substrate, wherein the first layers comprise an electrically insulating composite layer comprising a silicon nitride layer between silicon oxide layers and wherein the second layers comprise a semiconductor or conductor material; etching the stack to form a front side opening in the stack; forming a blocking dielectric layer over the stack of alternating first and second layers exposed in the front side opening; forming a charge storage layer over the layer of high work function material; forming a tunnel dielectric layer over the charge storage layer; forming a semiconductor channel layer over the tunnel dielectric layer; etching the stack to form a back side opening in the stack; removing at least a portion of the silicon nitride layer between silicon oxide layers to form back side recesses between adjacent second layers; removing portions of the blocking dielectric layer exposed in the back side recesses; and forming discrete charge storage regions, wherein at least one feature selected from a group consisting of six features is employed to make the monolithic three dimensional NAND string; and the group of six features consists of: a first feature wherein the charge storage layer comprises a semiconductor, metal or silicide charge storage layer and the method further comprises forming a layer of material having a work function higher than the semiconductor metal or silicide charge storage layer over the blocking dielectric layer prior to forming the charge storage layer; a second feature wherein the charge storage layer comprises intrinsic polysilicon and the charge storage regions comprise floating gates; a third feature wherein the step of forming the discrete charge storage regions comprises oxidizing the charge storage layer material exposed in the back side recesses, wherein the discrete charge storage regions comprise concave boundaries located perpendicular to the tunnel dielectric layer; a fourth feature wherein the step of forming the discrete charge storage regions comprises removing portions of the charge storage layer exposed in the back side recesses by etching; a fifth feature wherein wherein the layers of silicon oxide in the composite layer protect the second material when removing at least a portion of the silicon nitride layer between silicon oxide layers; and a sixth feature wherein the blocking dielectric layer comprises a silicon nitride layer located between a first silicon oxide layer and a second silicon oxide layer, and wherein the method further comprises: removing the first oxide layer and the nitride layer of the blocking dielectric layer and the silicon oxide layers of the composite layer after removing the silicon nitride layer of the composite layer; forming a protective layer on portions of the second material layers exposed in the back side recesses; removing the second oxide layer of the blocking dielectric; and forming the discrete charge storage regions by etching or oxidation of portions of the charge storage layer exposed in the back side recesses.
 19. The method of claim 18, wherein the first feature is employed to make the monolithic three dimensional NAND string.
 20. The method of claim 19, further comprising removing the layers of silicon oxide of the composite layer and the layer of higher work function material exposed in the back side recesses.
 21. The method of claim 19, wherein the layer of higher work function material comprises ruthenium or titanium nitride and the tunnel dielectric comprises silicon oxide.
 22. The method of claim 18, wherein the second feature is employed to make the monolithic three dimensional NAND string.
 23. The method of claim 18, wherein the third feature is employed to make the monolithic three dimensional NAND string.
 24. The method of claim 23, wherein a portion of the concave boundaries of the discrete charge storage regions comprise a bird's peak shape.
 25. The method of claim 18, wherein the fourth feature is employed to make the monolithic three dimensional NAND string.
 26. The method of claim 25, wherein the second layers comprise polysilicon or amorphous silicon doped with at least one of carbon or boron, the charge storage layer comprises intrinsic polysilicon, and the second material layers are etched at a lower rate than the charge storage layer during the step of forming the discrete charge storage regions.
 27. The method of claim 18, wherein the fifth feature is employed to make the monolithic three dimensional NAND string.
 28. The method of claim 18, wherein the blocking dielectric layer comprises a silicon nitride layer located between a first silicon oxide layer and a second silicon oxide layer.
 29. The method of claim 18, the sixth feature is employed to make the monolithic three dimensional NAND string.
 30. A method of making a monolithic three dimensional NAND string, comprising: forming a stack of alternating layers of a first material and a second material over a substrate, wherein the first material comprises an electrically insulating material and wherein the second material comprises a semiconductor or conductor material; etching the stack to form a front side opening in the stack; forming a blocking dielectric layer over the stack of alternating layers of a first material and a second material exposed in the front side opening; forming a charge storage layer over the blocking dielectric; forming a tunnel dielectric layer over the charge storage layer; forming a semiconductor channel layer over the tunnel dielectric layer; etching the stack to form a back side opening in the stack; removing at least a portion of the first material layers through the back side opening to form back side recesses between the second material layers; forming a protective layer on portions of the second material layers exposed in the back side recesses; after forming the protective layer, removing portions of the blocking dielectric layer exposed in the back side the recesses through the back side opening; and forming discrete charge storage regions.
 31. The method of claim 30, wherein the protective layer comprises a silicon nitride layer.
 32. The method of claim 30, wherein: the blocking dielectric layer comprises a silicon nitride layer located between a first silicon oxide layer and a second silicon oxide layer; and the first layers comprise an electrically insulating composite layer comprising a silicon nitride layer between silicon oxide layers.
 33. The method of claim 32, further comprising: removing the silicon nitride layer of the composite layer; before the step of forming the protective layer, removing the first oxide layer and the nitride layer of the blocking dielectric layer and the silicon oxide layers of the composite layer; after the step of forming the protective layer, removing the second oxide layer of the blocking dielectric; and forming the discrete charge storage regions by etching or oxidation of portions of the charge storage layer exposed in the back side recesses. 